perf: avoid copying already rooted request paths (#2794)

Preparing SCRIPT_FILENAME currently prepends `/` even when the request
path already starts with it. The resulting extra slash makes
`path.Clean` rebuild an otherwise clean path.

Only prepend `/` for unrooted paths. Keep path cleaning, the Windows
locality check, and trailing-slash handling. Add cases for repeated
leading slashes and traversal, plus a benchmark for rooted, relative,
and traversal paths.

Earlier standalone measurements of this helper (Go 1.27.3, darwin/arm64,
median of five 200ms samples):

| Path | Before | After | Time reduction | Go B/op | allocs/op |
| --- | ---: | ---: | ---: | ---: | ---: |
| `/index.php` | 111.8 ns | 69.69 ns | 37.7% | 80 → 32 | 4 → 1 |
| `/products/catalog/index.php` | 153.9 ns | 110.9 ns | 27.9% | 144 → 48
| 4 → 1 |
| Long rooted path | 499.2 ns | 413.3 ns | 17.2% | 656 → 176 | 4 → 1 |

These are helper-level results, not whole-request or RSS measurements.
Relative paths had a 2.4% time reduction with unchanged allocations;
traversal paths had an 11.0% reduction with Go B/op dropping from 96 to
72. The original benchmark cases were not rerun.

Validation:

- `gofmt` and `git diff --check` passed.
- The exact helper and its repository path tests passed with `-race` in
an isolated harness without PHP linkage.
- A differential fuzz run against the original helper passed 657,119
executions.
- The isolated tests cross-compiled for Windows; Windows runtime tests
were not run locally.
- Full FrankenPHP tests were not run: the local environment lacks the
required PHP ZTS/shared-library and watcher setup.


Adversarial benchmark cases use 4096 leading slashes and 512 rooted
`a/../` segments. Separate exploratory measurements (Go 1.27.3, Apple
M2, darwin/arm64; one 300ms sample per case):

| Input | Before | After | Go B/op | allocs/op |
| --- | ---: | ---: | ---: | ---: |
| 4096 leading slashes | 3260 ns | 2782 ns | 9776 → 4912 | 4 → 3 |
| 512 rooted `a/../` segments | 3533 ns | 3391 ns | 5424 → 2736 | 4 → 3
|

These exploratory measurements do not establish statistical significance
or an improvement on every input. Allocation counts and bytes do not
increase in these cases.

The branch also restores the `os` import needed by
`os.ErrDeadlineExceeded`, which was lost when main was merged and caused
the CI compilation failure.

Author: andreybolonin

cgi.go

@@ -345,7 +345,7 @@ func sanitizedPathJoin(root, reqPath string) string {
 
 	// Clean with "path", not "filepath": on Windows filepath.Clean keeps the
 	// ".." behind a leading "//" (UNC prefix), letting reqPath escape root.
-	relPath := path.Clean("/" + reqPath)[1:]
+	relPath := path.Clean(ensureLeadingSlash(reqPath))[1:]
 	// A cleaned rooted path is always local on POSIX: only Windows needs the check
 	if runtime.GOOS == "windows" && relPath != "" && !filepath.IsLocal(relPath) {
 		// path is unsafe (see https://github.com/golang/go/issues/56336#issuecomment-1416214885)

cgi_test.go

@@ -50,6 +50,9 @@ func TestSanitizedPathJoin(t *testing.T) {
 		expected string
 	}{
 		{"leading slash", "/index.php", filepath.Join(root, "index.php")},
+		{"multiple leading slashes", "///index.php", filepath.Join(root, "index.php")},
+		{"multiple root slashes", "//", filepath.Clean(root) + separator},
+		{"rooted nested traversal", "//foo/../../bar/", filepath.Join(root, "bar") + separator},
 		{"no leading slash", "index.php", filepath.Join(root, "index.php")},
 		{"nested", "/foo/bar/index.php", filepath.Join(root, "foo", "bar", "index.php")},
 		{"empty req path returns root", "", filepath.Clean(root)},